Experimental Geoelectric Field 1- minute

The Geoelectric Field is a measure of the induction hazard to artificial conductors, such as electrical power lines, that results from Geomagnetic Activity, and can be used to estimate the amount of current induced by integrating along the conducting pathway.

The near real-time E-field mapping project is a joint effort between NOAA/SWPC, the USGS geomagnetism group, and the NASA/CCMC.

Background

Potentially hazardous geoelectric fields can be induced during geomagnetic storms. These geomagnetic storms are a form of space weather driven by greatly enhanced currents in Earth's magnetosphere and ionosphere and are observed at ground level as a time-varying magnetic field. As is well known from Faraday's law, a time-varying magnetic field induces currents along natural and artificial conducting pathways. This geoelectric field product combines information about the time-varying magnetic field together with available Earth-conductivity information to estimate regional geoelectric field levels on a geographic grid over the lower 48 states. The amount of current induced in an artificial conductor may be calculated by integrating along the conducting pathway. When currents are induced in artificial conductors, unexpected and sometimes problematic effects can occur in the operation of the affected equipment. Please see our article about the effect this has on electrical power systems at http://www.swpc.noaa.gov/impacts/electric-power-transmission. Please see also the article Modeling geomagnetically induced currents, by Boteler and Pirjola in Space Weather (31 January 2017), for an up-to-date description of this phenomena.

Versions and Caveats:

The version of this product using 1D physiographic conductivity models is based on analysis published by Fernberg (2012). We also anticipate an upcoming release of 3D Geoelectric Field Maps in FY2018. The 3D Geoelectric Field Maps will be a version of the product that is based on the state-of-the-art three-dimensional (3D) electrical conductivity models obtained with magnetotellurics (Meqbel et al., 2014; Yang et al., 2015; Murphy & Egbert, 2017) using NSF's EarthScope MT magnetotelluric data set, and global electromagnetics (Sun et al., 2015), using USGS and worldwide geomagnetic observatories from INTERMAGNET. The upcoming 3D Geoelectric Field Maps will follow the same output format as the 1D Geoelectric Field Maps, and the two services will run side by side in the foreseeable future to facilitate scientific research and validation.

Our users should note specifically that 1D Geoelectric Field Maps, as currently released, are in need of validation against geoelectric field or geomagnetically induced current measurements. Recent research (e.g. Bedrosian & Love, 2015; Weigel, 2017; Bonner & Schultz, 2017; Kelbert et al., 2017) suggest that in some regions, the 1D approximation for the Earth's structure does not hold and the geoelectric field estimation in the 1D Geoelectric Field Maps could be substantially inaccurate. At this time, we advise against utilization of the 1D Geoelectric Field Maps for operational mitigation of geomagnetic hazards. We hope, however, that the release of this product will facilitate research on geomagnetic hazards and validation activities within the power-grid industry. Operational and research workflows developed with the use of the 1D Geoelectric Field Maps will be directly portable to the 3D Geoelectric Field Maps, once available. We invite development of such workflows, which would dramatically assist us with validation efforts.

Acknowledgements:

Key data provider agencies are gratefully acknowledged for their contributions:
-The U.S. magnetometer observatories are operated and maintained by the U.S. Geological Survey
-The near U.S. Canadian observatories are operated and maintained by NRCAN

The maps use a geomagnetic-field time series interpolation algorithm (Spherical Elementary Current Systems) developed and made available courtesy of the Finnish Meteorological Institute (Amm & Viljanen, 1999; Pulkkinen et al., 2003)

Results from the NSF's EarthScope USArray project are being used as the source for the development of greatly improved, 3D Earth conductivity models which will be incorpated in the 3D Geoelectric Field Maps as discussed above.

Technical advice from David Boteler, NRCAN, is gratefully acknowledged.

Local specification of the Geoelectric Field was identified by users in the electrical power industry as a critical need at SWPC's space weather workshop in 2011. Since then, through collaboration between SWPC, USGS, NASA/CCMC, and NRCAN, efforts have been devoted to meeting this important need. This parameter has also been identified as the key measure by the North American Electric Reliability Corporation in terms of Geomagnetic Disturbance mitigation. In particular, a benchmark Geomagnetic Disturbance Event has been defined and is being refined in terms of Geoelectric Field time series in order for the industry to carry out vulnerability assessments and mitigation measures. The quantity was also highlighted by the National Space Weather Action Plan from the Office of Science and Technology Policy of the President in the initial draft (October 2015) and through later versions of the plan.

The local geoelectric field is specified in millivolts per kilometer and is based on convolving a geomagnetic time-series signature with an Earth-response function, where the response function depends on the local Earth conductivity. In this initial 1D version, geomagnetic time-series are interpolated onto a 2 degree by 2 degree grid using the method of Spherical Elementary Current Systems (SECS - see Amm & Viljanen, 1999; Pulkkinen et al. 2003 for more information about the method). The Earth conductivity is determined, based on the physiographic region that the grid point lies in and the associated, one-dimensional conductivity profile as compiled by Fernberg (EPRI Technical Update 1026430, 2012). Active work is in progress by USGS and SWPC to replace these very simple conductivity models up-to-date Earth-Response functions that incorporate the full effects of the 3D Earth conductivity (see Product Details tab for more information).

Initial release of the 1D Geoelectric Field Maps (graphics) is scheduled for October 2017. Data values are available on request.

Archive maps and data for the 1D Geoelectric Field Maps are available by request.